1. Field of the Invention
The present invention generally relates to a fiber reinforced thermoplastic sheet-shaped molding which is made of reinforcement fibers impregnated with thermoplastic resin and which may be used as material for products of a type requiring a high resistance to heat and a high creep characteristic such as, for example, automobile component parts, component parts of electric appliances, daily commodities and so on.
The present invention also relates to a method of and an apparatus for manufacturing the fiber reinforced thermoplastic sheet-shaped molding of the kind referred to above.
2. Description of the Prior Art
The fiber reinforced thermoplastic sheet-shaped molding is not a recent development and has been used for many years in various products. As is well known to those skilled in the art, the fiber reinforced thermoplastic sheet-shaped molding is a thermoplastic sheet-shaped molding in which reinforcement fibers having a relatively large length are distributed to render it to exhibit an improved physical strength. Not only is the fiber reinforced synthetic resin used in numerous applications, the fiber reinforced thermoplastic resin is often used as an alternative to metal because of its high physical strength.
However, the prior art fiber reinforced thermoplastic sheet-shaped molding generally has a problem in that it has a high void content with voids being relatively large in size and is therefore relatively low in physical strength.
According to the prior art, the fiber reinforced thermoplastic sheet-shaped molding is manufactured by a number of methods. For example, the Japanese Laid-open Patent Publication No. 55-77525, published in 1980, (which is a Japanese counterpart of U.S. patent application Ser. No. 965,956 filed in 1978) discloses a method comprising steps of laminating a ply of reinforcement fibers and a ply of thermoplastic material one above the other, compressing the laminated plies together by the application of a pressure across the direction of thickness thereof while the plies are heated to cause the reinforcement fibers to be impregnated in the ply of thermoplastic material thereby to provide a fiber reinforced synthetic sheet, and cooling the resultant fiber reinforced thermoplastic sheet-shaped molding.
Another method known to those skilled in the art comprises steps of supplying thermoplastic resin in melt state between plies of reinforcement fibers to provide a resin layer of a sandwich structure, supplying thermoplastic sheets or films onto the layer of sandwich structure so as to form respective outermost layers, and passing the resultant layered structure through a belt-type impregnating machine, including a heating unit, a compressing unit and a cooling unit, thereby to cause the reinforcement fibers to be impregnated in the thermoplastic material.
The second mentioned method in which the belt-type impregnating machine is employed is available in two processes depending on the type of thermoplastic resin used. One of those processes employs, as the thermoplastic resin, polyester resin such as, for example, polyethylene terephthalate. An apparatus capable of practicing this process using the polyester resin is schematically illustrated in FIG. 14 of the accompanying drawings, reference to which will now be made for the detailed discussion thereof.
Referring to FIG. 14, a laminated web made up of sheets a of polyethylene terephthalate and a mat (b) of reinforcement fibers positioned intermediate between the polyethylene terephthalate (PET) sheets a is supplied in between a nipping region between upper and lower conveyor belts 1A and 1B made of heat-resistant metal and forming a belt-type impregnating machine 7. The upper and lower conveyor belts 1A and 1B are supported for movement in a direction of transport of the laminated web and in a direction close to each other so that pressure can be applied to the laminated web from opposite directions transverse to the laminated web.
The belt-type impregnating machine 7 also comprises a heating and compressing unit 4, which includes a plurality of heating rolls 3 adapted to be heated by respective heaters 2 and juxtaposed behind a feed run of each of the upper and lower conveyor belts 1A and 1B, and a cooling unit 6 positioned on one side downstream of the heating and compressing unit 4 with respect to the direction of transport of the laminated web (c) and including a plurality of cooling rolls 5 juxtaposed behind a feed run of each of the upper and lower conveyor belts 1A and 1B. Each of the cooling rolls 5 for each conveyor belt 1A and 1B is of a type having a longitudinal hollow through which a coolant such as, for example, water or oil, can flow in one direction.
This belt-type impregnating machine 7 is so designed and so operable that, during the transport of the laminated web, as indicated by (c), through the nipping region defined between the upper and lower conveyor belts 1A and 1B, the laminated web (c) can be compressed and heated by the heating and compressing unit 4 to melt the polyethylene terephthalate sheets (a) thereby to cause the latter to penetrate into interstices in the mat (b) of reinforcement fibers and is subsequently cooled by the cooling unit 6 while still compressed by the upper and lower conveyor belts 1A and 1B.
Although not shown, as a matter of practice, the upper and lower conveyor belts 1A and 1B are drivingly coupled with a common drive motor 8 through a train of gears so that the upper and lower conveyor belts 1A and 1B can run in respective directions opposite to each other at an equal speed to transport the laminated web (c) in a direction shown by the arrow x.
The other process employs the thermoplastic resin of a kind that requires no drying, examples of which include polyolephiline resin such as, for example, polypropylene or polymethyl pentene, or polyacethale resin or ABC resin. An apparatus capable of practicing this process using the thermoplastic resin of the kind which does not require any drying is schematically shown in FIG. 15 of the accompanying drawings, reference to which will now be made for the detailed discussion thereof.
Referring to FIG. 15, sheets (a) of thermoplastic resin and mats (b) of reinforcement fibers are supplied from rolls in alternating fashion with each other to form a continuous laminated web (c) which is subsequently transported into the nipping region defined in a belt-type impregnating machine of a type substantially identical with that shown in FIG. 15 for the manufacture of a continuous fiber reinforced thermoplastic sheet-shaped molding. This machine is disclosed in, for example, the Japanese Laid-open Patent Publication No. 61-279518, published in 1986.
However, the prior art fiber reinforced thermoplastic sheet-shaped molding has a relatively high void content with voids relatively large in size and has consequently a reduced physical strength.
According to the prior art methods of manufacturing the fiber reinforced thermoplastic sheet-shaped molding with the use of the belt-type impregnating machines 7, because the heating of the laminated web (c) containing the thermoplastic resin sheets (a) and the reinforcement fiber mats (b) is initiated substantially simultaneously with the application of the pressure thereto in a direction inwardly across the thickness thereof after it has been supplied into the nipping region in the impregnating machine 7 and, also, because the heating rolls 3 forming the heating and compressing unit 4 play an important role in heating the laminated web (c), a relatively long time is required for the laminated web (c) to be heated to an extent that the thermoplastic resin in melt state can penetrate sufficiently into interstices in the reinforcement fiber mats (b), constituting a cause of reduction in productivity of the intended fiber reinforced thermoplastic sheet-shaped molding. Also, the heating of the laminated web (c) for a substantially prolonged time can result in a thermal deterioration such as, for example, thermal decomposition to such an extent that the fiber reinforced thermoplastic sheet-shaped molding can no longer exhibit a satisfactory performance.
Furthermore, since the impregnation of the melted thermoplastic resin into the interstices in the reinforcement fiber mats (b) is carried out by the application of pressure thereto by means of the rolls 3 compressing it from opposite directions, it often occurs that the reinforcement fibers tend to displace in position under the influence of applied physical pressures and/or an uneven penetration of the melted thermoplastic resin into the interstices in the reinforcement fiber mats (b) tends to take place when the melted thermoplastic resin receives an instantaneous application of the physical pressures, that is, a localized pressure. These may constitute a cause of reduction in quality of the resultant fiber reinforced thermoplastic sheet-shaped molding.